Control rod guide tube and method for providing coolant to a nuclear reactor fuel assembly

ABSTRACT

Control rod guide tubes for a nuclear reactor having a body with an axial length that defines a lower end portion and an upper end portion and a cavity within a substantial length of the body. Orifices are included at the upper and lower end portions of the body. A control rod chamber is located within the cavity and is configured for receiving a control rod. A plurality of ports is coupled to the cavity and is positioned at a substantial length from the upper end portion of the body. Also included are at least two flow channels within the cavity that extend a substantial portion of the axial length of the body. Each flow channel is fluidly coupled to one or more of the ports for receiving fluid flow from outside the body and an outlet proximate to the upper end portion of the body for providing the received fluid flow.

PRIORITY STATEMENT

This application is a divisional application of U.S. patent applicationSer. No. 11/644,485, filed on Dec. 22, 2006 now U.S. Pat. No. 7,672,418(published as U.S. Patent Application Publication No. 2008/0152068 A1 onJun. 26, 2008), and claims the associated benefit under 35 U.S.C. §120and 35 U.S.C. §121. The entire contents of parent U.S. patentapplication Ser. No. 11/644,485, entitled “CONTROL ROD GUIDE TUBE ANDMETHOD FOR PROVIDING COOLANT TO A NUCLEAR REACTOR FUEL ASSEMBLY”, areincorporated herein by reference.

BACKGROUND

1. Field

The present disclosure relates to nuclear reactors and, morespecifically, control rod guide tubes for supporting control rodsextracted from the reactor core and for channeling coolant flow to fuelsupports and fuel assemblies in a reactor core.

2. Description of Related Art

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

A nuclear reactor pressure vessel (RPV) has a generally cylindricalshape and is closed at both ends, e.g., by a bottom head and a removabletop head. A top guide is spaced above a core plate within the RPV. Acore shroud, or shroud, surrounds the core plate and is supported by ashroud support structure. Particularly, the shroud has a generallycylindrical shape and surrounds both the core plate and the top guide.The top guide includes several openings, and fuel assemblies areinserted through the openings and are supported by the core plate. Thecore plate includes a flat plate supported by a plurality of beams.

A nuclear reactor core includes a plurality of individual fuelassemblies that have different characteristics that affect the strategyfor operation of the core. For example, a nuclear reactor core typicallyhas several hundred individual fuel assemblies that have differentcharacteristics; each fuel assembly includes a plurality of fuel rods.The fuel assemblies are arranged within the reactor core so that theinteraction between the fuel assemblies satisfies regulatory and reactordesign guidelines and constraints. In addition the core arrangementdetermines the cycle energy, which is the amount of energy that thereactor core generates before the core needs to be refreshed with newfuel elements, the core loading arrangement preferably optimizes thecore cycle energy.

A core cycle is determined from one periodic reactor core refueling to asecond reactor core refueling. During the course of the cycle ofoperation, the excess reactivity, which defines the energy capability ofthe core, is controlled in two ways. Specifically, a burnable poison,e.g., gadolinia, is incorporated in the fresh fuel. The quantity ofinitial burnable poison is determined by design constraints typicallyset by the utility and by the National Regulatory Commission (NRC). Theburnable poison controls most, but not all, of the excess reactivity. Asecond way is through the manipulation of control rods within the core.Control rods control the excess reactivity. Specifically, the reactorcore contains control rods which assure safe shutdown and provide theprimary mechanism for controlling the maximum power peaking factor. Thetotal number of control rods available varies with core size andgeometry, and is typically between 50 and 269. The position of thecontrol rods, i.e., fully inserted, fully withdrawn, or somewherebetween, is based on the need to control the excess reactivity and tomeet other operational constraints, such as the maximum core powerpeaking factor.

Coolant is introduced in the core to cool the core and to betransitioned into steam as a working fluid for energy generation. Normalcoolant flow enters the fuel assemblies as a single phased flow withslightly sub-cooled coolant from the fuel support. The flow goesvertically upward around the control rod guide tubes, and then turnshorizontally as the flow enters a side inlet to a fuel supportsupporting a fuel assembly. The flow then turns ninety degree within thefuel support and upward until it passes through an orifice of the fuelsupport to provide a pressure drop to assist coolant distribution to thefuel assemblies. The flow then turns vertical and enters a lumen on thelower tie plate of the fuel assembly and is distributed around theindividual fuel rods of the fuel assembly.

Known reactors have included fuel support orifice regions within thecore, one around the peripheral and one near the center. The peripheralregion includes all fuel locations around the periphery of the core, andthe center region includes the remainder of the locations. The fuelsupport orifices are designed to limit the fluid flow to the fuelassemblies in the peripheral region to about half of the fluid flow perfuel element of the center region. Limiting the peripheral flow by thismagnitude has permitted the very low power peripheral fuel elements tosaturate the coolant flow, but with maintaining the exit quality andaverage voids that are still much lower than for the other higher powerregion. This uneven exit quality and average void can produceinefficient steam separation and nuclear moderation.

It is also known that the coolant flow can be adjusted through varyingthe design of the fuel assembly. For example, it is known that each fuelassembly can include a main coolant flow channel and inlet that has asubstantial constant flow. However, the fuel assemblies can also includeone or more secondary coolant flow channels that can vary to adjust thecoolant flow in the particular fuel assemblies. In some cases, threetypes of fuel assemblies can provide three different secondary coolantflows. Each such fuel assembly can be positioned in the core to providefor a desired coolant flow. For example, the three different fuelassemblies can be arranged into three or more core regions. The flow ofcoolant through each fuel assembly in each region can be different fromthe coolant flow through a fuel assembly in each other region based onthe position of the three different fuel assemblies. However, thisrequires the manufacture of three different fuel assemblies and/or tieplates.

In the known reactor arrangements, the fluid flow into the fuel supportand then into the lower tie plate of the fuel assemblies is asymmetricaland unstable.

SUMMARY

The inventors hereof have succeeded at designing control rod guide tubesthat can enable an improved symmetrical and/or stable fluid flow intothe fuel support and then into the fuel assembly. Additionally, theinventors hereof have designed a reactor core fluid flow assembly andmethods for providing coolant into fuel assemblies having a reducedpressure drop associated with the providing of the fluid flow to thefuel support and, therefore, to fuel assemblies.

According to one aspect, a control rod guide tube for a nuclear reactorincludes a body having an axial length defining a lower end portion andan upper end portion and a cavity within a substantial length of thebody including orifices at the upper and lower end portions of the body.A control rod chamber located within the cavity is configured forreceiving a control rod. A plurality of ports is coupled to the cavityand is positioned at a substantial length from the upper end portion ofthe body. Also included are at least two flow channels within the cavityextending a substantial portion of the axial length of the body. Eachflow channel is fluidly coupled to one or more of the ports forreceiving fluid flow from outside the body and an outlet proximate tothe upper end portion of the body for providing the received fluid flow.

According to another aspect, a control rod guide tube for a nuclearreactor includes a body having a cylindrical wall defining an upper endportion, a lower end portion, a cavity defined by an interior surface ofthe wall and extending from the upper end portion to the lower endportion, and a plurality of ports positioned axially along the wallbetween the upper end portion and the lower end portion for providingfluid flow into the body cavity. Also included is an insert dimensionedfor positioning within the body cavity and having an upper end portionand a lower end portion and including a control rod chamber adapted forreceiving a control rod and a plurality of channel fixtures that, atleast partially, define one or more flow channels within the cavity ofthe body. The flow channels are configured for receiving a fluid flowthrough one or more of the body ports, channeling the received fluidflow within the body cavity between the lower end portion and the upperend portion, and providing the fluid flow to the upper end portion ofthe body.

According to yet another aspect, a control rod guide tube for a nuclearreactor includes means for receiving a control rod, and means forchanneling a substantially symmetrical fluid flow into a lower orificeof a fuel assembly cavity of a fuel support.

According to still another aspect, a method of stabilizing fluid flowsto fuel assemblies within a nuclear reactor includes enclosing a controlrod chamber within a cavity of a body of a control rod guide tube. Thecontrol rod chamber is adapted for receiving a control rod. A pluralityof axial flow channels are positioned within the body cavity of thecontrol rod guide tube. The method also includes coupling the body to afuel support that has a plurality of fuel assembly cavities adapted forproviding the fluid flows to the fuel assemblies. The coupling includesfluidly mating each of the axial flow channels to a corresponding fuelassembly cavity.

According to another aspect, a method of flow control management in anuclear reactor includes receiving a fluid flow into a flow channel of acontrol rod guide tube through one or more ports defined by the controlrod guide tube, providing the received fluid flow from the flow channelto a cavity of a fuel support, providing the fluid flow from the fuelsupport cavity to a lumen on a lower tie plate of a fuel assembly.

Further aspects of the present invention will be in part apparent and inpart pointed out below. It should be understood that various aspects ofthe disclosure may be implemented individually or in combination withone another. It should also be understood that the detailed descriptionand drawings, while indicating certain exemplary embodiments, areintended for purposes of illustration only and should not be construedas limiting the scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cutaway side view of a nuclear reactor operating environmentsuitable for some exemplary embodiments.

FIG. 2 is a side perspective view of a control rod guide tube bodyaccording to one exemplary embodiment.

FIG. 3 is a side perspective view of a multi-component insert havingfour insert fixtures according to one exemplary embodiment of a controlrod guide tube.

FIG. 4 is a side perspective view of a monolithic insert according toanother exemplary embodiment of a control rod guide tube.

FIG. 5 is an end perspective view of a control rod guide tube accordingto one exemplary embodiment.

FIG. 6 is a bottom perspective view of a fuel support suitable forcoupling to some control rod guide tube embodiments of the presentdisclosure.

FIG. 7 is a bottom view of a partially disassembled control rod guidetube with a coupled fuel support illustrating alignment of the controlrod guide tube insert with the fuel support according to one exemplaryembodiment.

FIG. 8 is a side perspective view of an assembly including one exemplarycontrol rod guide tube, a coupled fuel support and an affixed couplingfixture according to one exemplary embodiment.

It should be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The following description is merely exemplary in nature and is notintended to limit the present disclosure or the disclosure'sapplications or uses.

In some embodiments, a control rod guide tube for a nuclear reactorincludes a body having an axial length defining a lower end portion andan upper end portion and a cavity within a substantial length of thebody including orifices at the upper and lower end portions of the body.A control rod chamber located within the cavity is configured forreceiving a control rod. A plurality of ports is coupled to the cavityand is positioned at a substantial length from the upper end portion ofthe body. Also included are at least two flow channels within the cavityextending a substantial portion of the axial length of the body. Eachflow channel is fluidly coupled to one or more of the ports forreceiving fluid flow from outside the body and an outlet proximate tothe upper end portion of the body for providing the received fluid flow.This can be better understood with reference to the figures.

As seen by way of the exemplary operating environment of FIG. 1, aconventional boiling water reactor (BWR) has a reactor pressure vessel10 and a core shroud 12 arranged concentrically in the reactor pressurevessel 10 with an annular region, namely, the downcomer annulus 14,therebetween. The core shroud 12 is a stainless steel cylindersurrounding the nuclear fuel core 13. In particular, the core shroud 12comprises a shroud head flange 12 a for supporting the shroud head (notshown); a circular cylindrical upper shroud wall 12 b having a top endwelded to shroud head flange 12 a; an annular top guide support ring 12c welded to the bottom end of upper shroud wall 12 b; a circularcylindrical middle shroud wall 12 d that is a welded assembly welded tothe top guide support ring 12 c; and an annular core plate support ring12 e welded to the bottom of the middle shroud wall 12 d and to the topof a lower shroud wall 12 f. As seen in FIG. 1, the shroud 12 isvertically supported by a plurality of shroud support legs 16, each ofthe latter being welded to the bottom head 17 of the reactor pressurevessel 10. The core shroud 12 is laterally supported by an annularshroud support plate 18, which is welded at its inner diameter to thecore shroud 12 and at its outer diameter to the reactor pressure vessel10. The shroud support plate 18 has a plurality of circular apertures 20in flow communication with diffusers of a plurality of jet pumpassemblies (not shown),

The fuel core 13 of a BWR consists of a multiplicity of upright andparallel fuel assemblies 22 (also referred to as fuel bundles) arrangedin arrays, each fuel assembly 22 includes an array of fuel rods inside afuel channel made of zirconium-based alloy. Each array of fuel bundleassemblies is supported at the top by a top guide 24 and at the bottomby a core plate 26 and its underlying support structure 27. The coreplate 26 subdivides the reactor into the fuel core 13 and a lower plenum15. The core top guide 24 provides lateral support for the top of thefuel assemblies 22 and the core plate 26 provides lateral support forthe bottom of the fuel assemblies 22. This lateral support maintains thecorrect fuel channel spacing in each array to permit vertical travel ofa control rod 28 including a plurality of control rod blades 29 betweenthe fuel assemblies 22.

The power level of the reactor is maintained or adjusted by positioningthe control rods 28 up and down within the core 13 while the fuelassemblies 22 are held stationary. Each control rod 28 has a cruciformcross-section consisting of four wings or control rod blades 29 at rightangles. Each control rod blade 29 consists of a multiplicity of paralleltubes welded in a row with each tube containing stacked capsules filledwith neutron-absorbing material. Each control rod 28 is raised orlowered with the support of a control rod guide tube 30 by an associatedcontrol rod drive (33) which can be releasably coupled by a spud at itstop to a socket in the bottom of the control rod 28.

The control rod drives 33 are used to position control rods 28 in a BWRto control the fission rate and fission density, and to provide adequateexcess negative reactivity to shutdown the reactor from any normaloperating or accident condition at the most reactive time in core life.Each control rod drive 33 is mounted vertically in a control rod drivehousing 32 which is welded to a stub tube 34, which in turn is welded tothe bottom head 17 of the reactor pressure vessel 10. The control roddrive 33 is a double-acting, mechanically latched hydraulic cylinder.The control rod drive 33 is capable of inserting or withdrawing acontrol rod (28) at a slow controlled rate for normal reactor operationand of providing rapid control rod 28 insertion (scram) in the event ofan emergency requiring rapid shutdown of the reactor.

The control rod drive housing 32 has an upper flange that bolts to alower flange of the control rod guide tube 30. Each control rod guidetube 30 sits on top of and is vertically supported by its associatedcontrol rod drive housing 32. The uppermost portion of the control rodguide tube 30 penetrates a corresponding circular aperture in the coreplate 26. There can be more than 140 control rod guide tubes 30penetrating an equal number of circular apertures 35 in the core plate26, each aperture 35 has a diameter slightly greater than the outerdiameter of the control rod guide tube 30.

The control rod drive housings 32 and control rod guide tubes 30 havetwo functions: (1) to house the control rod drive 33 mechanisms and thecontrol rods 28, respectively, and (2) to support the weight of the fuelin the fuel assemblies 22. The fuel weight is reacted at an orifice of afuel support 36 that is positioned on the top of the control rod guidetube 30. The control rod guide tubes 30 and housings 32 act as columnscarrying the weight of the fuel.

During operation of the reactor, water in the lower plenum 15 entersports 38 of the control rod guide tube 30. The water is channeled withinthe control rod guide tube 30 to the orifice of the fuel support 36 andinto a lumen of a lower tie plate of the fuel assemblies 22. The watercontinues to rise in the fuel assemblies 22 and in the fuel core 13,with a substantial amount turning to steam, which is used in theproduction of electrical energy.

As illustrated by the exemplary embodiments if FIGS. 2-5, the controlrod guide tube 30 has a body 40 with an axial length defining an upperend portion 42 and a lower end portion 44 and a cavity 46 within asubstantial length of the body 40. An orifice 48 at the upper endportion 42 and orifice 50 at the lower end portion 44 of the body 40.The upper end portion 42 can be adapted for coupling to a bottom of afuel support 36 for fluidly coupling a flow channel to a bottom orificeof the fuel support 36 and to a fuel assembly 22 engaged with orpositioned on top of the fuel support 36. The lower end portion 44 canbe adapted for coupling to a control rod drive housing 32 for supportingthe control rod guide tube 30 within the lower plenum 15 and inalignment with the control rod drive 33. This can include a couplingfixture (shown in FIG. 6) that can be attached to the lower end portionof the body 40 for releasably coupling the control rod guide tube 30.

A control rod chamber 52 (as shown in FIGS. 3, 4, and 5, by way ofexamples) is located within the cavity 46. The control rod chamber 52 isadapted, configured and/or dimensioned for receiving the control rod 28.As the control rod 28 generally has a cruciform shape, the control rodchamber 52 can also have a corresponding cruciform shape. The controlrod chamber 52 can be defined, at least in part, within the cavity 46 byone or more structures, referred herein generally as an insert 54. Theinsert can be have monolithic body or can be a grouping of one or moreinsert components that together form the insert 54 and that define, atleast in part, the control rod chamber 52 within the cavity 46.

One example of a multiple component insert 54A is illustrated, by way ofexample, in FIG. 3. In this embodiment, the insert 54A includes fourinsert flow fixtures 56 each having a curved-shaped that when assembledwith their convex portions back-to-back define a cruciform-shapedcontrol rod chamber 52. Additionally, each insert flow fixture 56defines a portion of a flow channel 58 by its convex shape. In someembodiments, each pair of insert flow fixtures 56 (also referred hereinas channel fixtures) are coupled at an outer periphery forming a hollowarm 61 defining a portion of the control rod chamber 52 configured forreceiving a control rod blade 29 of the control rod 30.

An example embodiment of a monolithic body for an insert 54B isillustrated in FIG. 4. In this embodiment, the cruciform-shaped controlrod chamber 52 is fully enclosed at the ends of each blade 29. Flowchannels 58 are also provided along an external convex surface of thisembodiment of insert 54B. Optionally, one or more control rod inlets 60can provide for a flow of coolant into the control rod chamber 52 andtherefore about the control rod 28 and its control rod blades 29contained with the control rod chamber 52.

The ports 38 are coupled to the cavity 46 and are positioned at asubstantial length from the upper end portion 42 of the body 40.Generally, a substantial length as described herein includes a length ofthe total substantial body length such that the flow from the ports 38to the upper end portion 42 within the cavity 46 becomes stable or isotherwise generally symmetrical, or lacking significant amounts ofasymmetries or turbulence. The substantial length can be proximate tothe lower end portion 44 as illustrated in FIG. 2, or can be at anydistance greater than near or proximate to the upper end portion 42. Assuch, a substantial length can include any length greater than a minorlength and is not intended to be indicated of requiring a majority ormore of the total length of the body 40.

Additionally, while FIGS. 2 and 5 illustrate five ports 38 positionedalong four sides of the body 40, more or less ports are possible andstill within the scope of this disclosure. Additionally, thecross-sectional area of the ports 38 can vary, as well as the number ofports axially aligned along the body 40.

As noted, the control rod guide tube 30 includes at least two flowchannels 58 within the cavity 46. In some embodiments, the flow channels58 are defined in part by the insert 54 and in part by an interiorsurface of the body 40. Generally, in some embodiments, the flowchannels 58 extend a substantial portion of the axial length of the body40. Each flow channel 58 is fluidly coupled to one or more ports 38 forreceiving fluid flow from the lower plenum 15 and an outlet 62 proximateto and/or defined by the upper end portion 42 of the body 40. The outlet62 provides the fluid flow to an orifice of a coupled fuel support 36.Generally, in some embodiments, the cross-sectional area of each flowchannel 58 is about equal to or less than a cross-sectional area of thecoupled fuel assembly orifice (not shown). In one embodiment, aplurality of ports 38 are coupled to a flow channel 58 and the combinedcross-sectional area of the coupled ports 38 is greater than across-sectional area of the coupled flow channel 58. In this manner,flow from the lower plenum 15 into the flow channel 58 is not restrictedat the ports 38 and turbulence can be reduced.

It should be noted that in some embodiments, the insert 54 can be fixedin position relative to the body 40. For example, the insert flowfixture 56 of insert 54A can be welded or otherwise affixed within thecavity 46 such as to an inner surface defining the cavity 46. This caninclude fixedly attaching the insert 54 to an inner surface such thateach flow channel 58 is substantially enclosed by a portion of the innersurface and the insert 54, to reduce any turbulence that can be causedor related to a non-enclosed or open or unattached portion of the insert54 and the inner surface.

Additionally, the monolithic insert 54B can also be affixed within thecavity 46 of the body 40. In other embodiments, the insert 54 can berotatable within the cavity 46. Having a rotatable insert 54 can providefor, among others, for rotating a control rod 38 within the control rodguide tube 30 during a refueling operation without having to remove thecontrol rod 38 and/or the control rod guide tube 30.

Referring now to FIG. 6, one embodiment of a fuel support 36 isillustrated from a bottom perspective. As shown, generally the fuelsupport 36 also includes a cruciform chamber for allowing passage of thecontrol rod 28 into the fuel core 13. The fuel support 36 includes aplurality of orifices 66 for receiving the flow from the flow channels38 of the control rod guide tube 30. The lower end portion 68 of thefuel support 36 is adapted for coupling to the upper end portion 42 ofthe body 40. This can include by welding or any other suitable method ofattachment. As shown in FIG. 7, the fuel support 36 and the control rodguide tube 30 are coupled to align the flow channel 58 with the orifices66. As shown, the insert flow fixtures 56 are aligned to define a flowchannel 58 that provides a fluid flow into each orifice 66. As notedabove, the cross-sectional area of the flow channels 58 can be aboutequal to or less than the cross-sectional area of the coupled orifice66. In such embodiments, little to no pressure increase occurs at thepoint of interface between the flow channel 58 and the orifice 66. Insome embodiments, the comparative cross-sectional areas can provide fora pressure drop at this interface.

FIG. 8 illustrates one embodiment of a control rod guide tube 30assembled with a fuel support 36 affixed to the upper end portion 42 anda coupling fixture 70 affixed to the lower end portion 44.

According to other embodiments, a method of stabilizing fluid flows tofuel assemblies within a nuclear reactor includes enclosing a controlrod chamber within a cavity of a body of a control rod guide tubewherein the control rod chamber is adapted for receiving a control rod.The method also includes defining a plurality of axial flow channelswithin the body cavity of the control rod guide tube. The method furtherincludes coupling the body to a fuel support having a plurality of fuelassembly cavities adapted for providing the fluid flows to the fuelassemblies. The coupling includes fluidly mating each of the axial flowchannels to a corresponding fuel assembly cavity.

This can include providing one or more ports on the body for each axialflow channel and/or defining axial flow channels to have across-sectional area less than or equal to a fluidly-mated fuel assemblycavity.

In another operational embodiment, a method of flow control managementin a nuclear reactor includes receiving a fluid flow into a flow channelof a control rod guide tube through one or more ports defined by thecontrol rod guide tube. The method also includes providing the receivedfluid flow from the flow channel to a cavity of a fuel support,providing the fluid flow from the fuel support cavity to a lumen on alower tie plate of a fuel assembly. Generally, this can includereceiving the fluid flow from one or more of the ports for reducing flowasymmetries within the flow channel and flow asymmetries as providedfrom the flow channels to an orifice of the fuel assembly coupledthereto.

In some embodiments, the method can also include providing a fluid flowfrom a flow channel to the fuel support orifice or cavity such that theprovided fluid flow does not experience or result in an increase (and insome embodiments within a substantial increase) in fluid pressure asprovided by control rod guide tube. This can be an improvement fromfluid flows provided by the traditional inlets on the sides of the fuelsupport. In this exemplary manner, a fuel support or other fluidhandling portions of the reactor can be modified to take advantage ofthe reduction in the fluid pressure provided by the control rod guidetube as described by the various embodiments of this disclosure.

This can include, in some embodiments, reducing a pressure drop of thefluid flow across the control rod guide tube as provided herein andincreasing a pressure drop of the fluid flow across the fuel support andthe lower tie plate as a result of the reduction in the inlet fluid flowas provided by the control rod guide tube as compared to the side inletof the fuel support. Further operational benefits for a reactor can beprovided by configuring or modifying the fuel support or an orifice orcavity thereof to further modify the fluid flow to the fuel assembly.

When describing elements or features and/or embodiments thereof, thearticles “a”, “an”, “the”, and “said” are intended to mean that thereare one or more of the elements or features. The terms “comprising”,“including”, and “having” are intended to be inclusive and mean thatthere may be additional elements or features beyond those specificallydescribed.

Those skilled in the art will recognize that various changes can be madeto the exemplary embodiments and implementations described above withoutdeparting from the scope of the disclosure. Accordingly, all mattercontained in the above description or shown in the accompanying drawingsshould be interpreted as illustrative and not in a limiting sense.

It is further to be understood that the processes or steps describedherein are not to be construed as necessarily requiring theirperformance in the particular order discussed or illustrated. It is alsoto be understood that additional or alternative processes or steps maybe employed.

1. A method of flow control management in a nuclear reactor, the methodcomprising: positioning a control rod guide tube in a lower plenum ofthe nuclear reactor, defining a flow channel and one or more ports inthe control rod guide tube; fluidly coupling an upper end portion of thecontrol rod guide tube to a fuel support; defining a cavity in the fuelsupport: fluidly coupling the fuel support to a lumen on a lower tieplate of a fuel assembly: receiving a fluid flow into the flow channelof the control rod guide tube through the one or more ports; providingthe received fluid flow from the flow channel to the cavity of the fuelsupport; and providing the fluid flow from the fuel support cavity tothe lumen on the lower tie plate.
 2. The method of claim 1, whereinreceiving the fluid flow into the flow channel includes receiving thefluid flow from the one or more ports defined by the control rod guidetube for reducing flow asymmetries.
 3. The method of claim 1, whereinthe one or more ports are positioned at a substantial distance from thefuel support.
 4. The method of claim 1, wherein providing the receivedfluid flow from the flow channel to the fuel support cavity includesensuring that a cross-sectional area of the flow channel is less than orequal to a cross-sectional area of the fuel support cavity to minimize apressure increase of the fluid flow between the flow channel and thefuel support cavity.
 5. The method of claim 1, wherein the defining ofthe flow channel and the one or more ports includes ensuring that theone or more ports has a total cross-sectional area greater than across-sectional area of the flow channel, and wherein thecross-sectional area of the flow channel is less than or equal to across-sectional area of the fuel support cavity.
 6. The method of claim1, wherein the defining of the flow channel and one or more portsfurther comprises: defining a plurality of flow channels in the controlrod guide tube; reducing a pressure drop of the fluid flow across thecontrol rod guide tube by providing a plurality of ports in the controlrod guide tube, for each flow channel.
 7. The method of claim 1, furthercomprising: configuring the fuel support cavity to modify the fluid flowthrough the fuel support cavity.
 8. The method of claim 1, wherein thecontrol rod guide tube includes: a body having an axial length defininga lower end portion and the upper end portion of the control rod guidetube; a cavity within a substantial length of the body including one ormore orifices at the upper and lower end portions of the body; a controlrod chamber within the cavity for receiving a control rod; a pluralityof ports coupled to the cavity and positioned at a substantial lengthfrom the upper end portion of the body; and at least two flow channelswithin the cavity extending a substantial portion of the axial length ofthe body.
 9. The method of claim 8, wherein the control, rod chamber hasa cruciform shape.
 10. The method of claim 8, wherein each flow channelis fluidly coupled to one or more of the ports for receiving fluid flowfrom outside the body, and wherein each flow channel is fluidly coupledto an outlet proximate to the upper end portion of the body forproviding the received fluid flow.
 11. The method of claim 8, whereinthe upper end portion of the body is configured to couple to a bottom ofa fuel support for fluidly coupling each of the flow channels to anorifice of a fuel assembly cavity defined by the fuel support.
 12. Themethod of claim 11, wherein a cross-sectional area of each flow channelis less than or equal to a cross-sectional area of the coupled orificeof the fuel assembly cavity.
 13. The method of claim 10, wherein acombined cross-sectional area of the ports coupled to each flow channelis greater than a cross-sectional area of the coupled flow channel. 14.The method of claim 8, further comprising: an insert positioned withinthe cavity; wherein the insert defines the control rod chamber withinthe cavity, and wherein the insert defines each of the flow channels inconjunction with an inner surface of the body defining the cavity. 15.The method of claim 14, wherein the insert is rotatable within thecavity.
 16. The method of claim 14, wherein the insert is fixedlyattached to the inner surface of the body.
 17. The method of claim 1,wherein the control rod guide tube includes: a body having a walldefining the upper end portion, a lower end portion, a cavity defined byan inner surface of the wall and extending from the upper end portion tothe lower end portion, and a plurality of ports positioned axially alongthe wall between the upper end portion and the lower end portion forproviding fluid flow into the cavity; and an insert dimensioned forpositioning within the cavity, the insert having an upper end portionand a lower end portion and including a control rod chamber configuredto receive a control rod and a plurality of channel fixtures that, atleast partially, define one or more flow channels within the body cavityfor receiving a fluid flow through one or more of the body ports; forchanneling the received fluid flow within the body cavity between thelower end portion and the upper end portion, and for providing the fluidflow to the upper end portion of the body.
 18. The method of claim 17,wherein each flow channel is configured to receive fluid flow from twoor more ports.
 19. The method of claim 17, wherein the insert is fixedlyattached to the inner surface of the wall, and wherein each flow channelis substantially enclosed by a portion of the inner surface and aportion of the insert.
 20. The method of claim 17, wherein the insert isrotatable within the body cavity.